The purpose of this project is to determine the natural history of the recently discovered and isolated causative agent of Lyme disease and related disorders. The relationship(s) between the spirochete and its various tick vectors (Ixodes dammini, I. pacificus, I. ricinus, Amblyomma americanum) is being determined by establishing through conventional as well as transmission and scanning electron microscopy (a) the development of the spirochete within the ticks, and (b) the mode(s) of transmission to vertebrate hosts. In cooperation with outside agencies, tick/spirochete surveys are being conducted to determine prevalence of infected ticks in endemic foci. Similarly, the natural source(s) for infecting ticks is being evaluated first serologically (indirect immunofluorescence) and subsequently through recovery of spirochetes from serologically implicated hosts. Of 838 Ixodes pacificus collected in California, 9 (1%) proved infected with spirochetes morphologically and antigenically indistinguishable from the Lyme disease spirochete isolated from I. dammini in eastern U.S. or from I. ricinus from Europe. So far, there is no evidence of spirochetes in the lone star tick, Amblyomma americanum from the TVA Land Between the Lake recreational area. New Zealand white rabbits are susceptible to the Lyme disease spirochete and are useful sources for the experimental infection of ticks with this agent. Of 98 Minnesota patients with tentative diagnosis of Lyme disease, 33 (33%) had IFA titers equal to or greater than 1:64 against a human isolate of the Lyme disease spirochete. Serologic results suggest that tick-borne meningoradiculitis (Bannwarth's syndrome) is a clinical expression of the Lyme disease spirochete.